1. Field of the Invention
The present invention relates to a polarization conversion optical system for converting light having a nonuniform plane of polarization to light having a uniform plane of polarization, and a polarization conversion element integrating the polarization conversion optical system in a single element.
2. Description of the Related Art
In optical devices using direct polarized light having a uniform plane of polarization, light having a nonuniform plane of polarization is converted to light having a uniform plane of polarization to increase the efficient use of the light. For example, in a projection image display device which modulates illumination light by a liquid crystal display device, nonpolarized light emitted from the lamp of a light source is converted to light having a uniform plane of polarization, and the converted light is directed to a liquid crystal display as illumination light to provide a bright projection image.
FIG. 10 shows a representative structure of a polarization conversion optical system for converting light having a nonuniform plane of polarization to light having a uniform plane of polarization. The polarization conversion optical system 50 comprises a polarization beam splitter (PBS) prism 51, half-wavelength plate 52, and mirror 53. The PBS prism 51 is produced by interposing a polarization beam-splitting film 51a between two triangular prisms. The polarization beam-splitting film 51a is a dielectric multilayer film set so as to transmit a p-polarized light component and reflect an s-polarized light component, thereby separating the p-polarized light component and s-polarized light component of mutually intersecting planes of polarization.
The half-wavelength plate 52 is disposed in the optical path of either of the linear polarized light of the separated p-polarized light component or the linear polarized light of the s-polarized light component, such that the plane of polarization of the impinging linear polarized light is rotated 90° to match the plane of polarization of the other linear polarized light. FIG. 10 shows a case wherein the p-polarized light component is transformed into an s-polarized light component relative to the polarization beam-splitting film 51a. 
Polarization beam splitting may be accomplished by using a PBS prism with an adhered doubly refracting crystal, rather than a PBS prism provided with a polarization beam-splitting film, in a polarization conversion optical system wherein the plane of polarization of one separated linear polarized light is rotated by a half-wavelength plate.
In recent years, polarization conversion optical systems have been proposed wherein polarization beam splitting is accomplished using a birefringent diffraction grating, and the plane of polarization is made uniform by a half-wavelength plate (e.g., Japanese Laid-Open Patent Application Nos. H10-197827, 2000-137194). The birefringent diffraction grating accomplishes polarization beam splitting by using the difference in the diffraction efficiency of the diffraction grating relative to the two polarized light components of mutually intersecting planes of polarization.
The structure of a birefringent diffraction grating is shown in FIG. 11. A birefringent diffraction grating 61 is produced by interposing a birefringent material 62 such as a liquid crystal having birefringence between a flat plate-like member 64 and a plate-like member 63 having a diffraction grating 63 formed thereon. The refractive indices of the birefringent material 62 and the member 63 are set so as to be equal relative to normal light o, and the refractive indices of the birefringent material 62 and the member 63 are different relative to abnormal light e. Accordingly, abnormal light e is diffracted and deflected by the diffraction grating 63a, and normal light o is not diffracted and advances rectilinearly. In this way both polarized light components are separated.
Both of the aforesaid polarization conversion optical systems are capable of setting the entirety of the polarized light to linearly polarized light having a uniform plane of polarization, whatever the polarization state of the conversion target light. Accordingly, optical devices provided with these polarization conversion optical systems are much more efficient at using light from a light source.
Both of these polarization conversion optical systems have a problem in that it is difficult to make them compact. In the polarization conversion optical system 50 shown in FIG. 10, since polarized beam splitting is accomplished by a regular quadrilateral pyramid-shaped PBS prism 51, the size in the direction along the beam prior to conversion is greater than the beam diameter. Since, directly after splitting, the two linear polarized light components advance in mutually perpendicular directions, a mirror is required to bend one of the optical paths, and the size in the direction perpendicular to the light beam is more than double the beam diameter.
In a polarization conversion optical system using a PBS prism with an adhered doubly refracting crystal and a polarization conversion optical system using the birefringent diffraction grating shown in FIG. 11, the size in the direction along the beam prior to conversion is larger than the polarization conversion optical system 50. Although the element used to split the polarized light is a thin element, there is no great angular difference in the direction of travel of the beams of the two polarized light components that pass through the element, such that a long optical path is required for complete separation of both polarized light components. The size in a direction perpendicular to the beams prior to conversion is more than double the beam diameter.
These polarization conversion optical systems using birefringence can be made compact to a certain degree if assembled with a micro lens array. As an example, FIG. 12 shows the structure of a polarization conversion optical system 60 in which a micro lens array 65 is combined with the birefringent diffraction grating 61 of FIG. 11. The micro lens array 65 is adhered to the exit surface of the birefringent diffraction grating 61, such that light transmitted through the birefringent diffraction grating 61 is condensed to individual convergent beams by the micro lenses 65a. The two polarized light components having different directions of travel converge at different positions, and are split at an early stage due to the convergent beams. A half-wavelength plate 66 is divided into equal numbers with the micro lenses 65a, which are arranged near the convergence position of the beam of one polarized light component.
In the polarization conversion optical system 60, the size in a direction perpendicular to the light beam prior to conversion is less than double the beam diameter, and the size in a direction along the light beam prior to conversion is also smaller. However, a half-wavelength plate 66 must be disposed at a position approximately a beam diameter from the micro lens array 65, thereby limiting the compactness of the entire optical system. Moreover, the assembly process becomes complex because the relative positions of the micro lens array 65 and the half-wavelength plate 66 must be precisely set.
In polarization conversion optical systems using a PBS prism with an adhered doubly refracting crystal, there are disadvantages relating to production characteristics because the doubly refracting crystal is expensive, and the process of adhering the doubly refracting crystal is complex.